Rate-responsive, distributed-rate pacemaker

ABSTRACT

A rate-responsive heart pacer in which rate-control parameter (RCP) values are arranged in a percentile ranking and mapped onto a percentile ranking of a desired rate distribution. By monitoring the RCP values over an extended time interval and developing a corresponding percentile ranking, the pacemaker automatically self-adapts to long-term changes in RCP measurements and insures that the desired rate distribution is obtained.

This is a division of application Ser. No. 150,037, filed Jan. 29, 1988now U.S. Pat. No. 4,856,522.

This invention relates to rate-responsive pacemakers, and moreparticularly to a rate-responsive pacemaker which exhibits apredetermined rate distribution independent of the distribution of therate-control parameter.

A rate-responsive pacemaker is one which adjusts its rate in accordancewith the value of a measured parameter. Because the value of theparameter is used to control the rate, it is generally referred to as arate-control parameter (RCP). The RCP varies with the physiologicalneeds of the body and is dependent upon such factors as stress, andwhether the patient is exercising or at rest. Illustrative rate-controlparameters include respiratory minute volume, QT interval, temperatureand physical vibration. A rate-responsive pacemaker generally exhibitssome characteristic which expresses the desired rate as a function ofthe RCP. Where the rate control is based upon such a built-incharacteristic, there are necessarily several disadvantages.

One of the disadvantages is that the RCP value for any given state ofstress or exercise does not remain constant for the life of thepacemaker. In some cases the RCP is measured by a sensor attached to apacemaker lead, or it is derived from the sensed electrogram signal. Ineither case, if the lead changes position, all values of the RCP may beshifted. If there is no way to account for shifts of this kind, it ispossible for all pacing rates to be shifted downwardly or upwardly.

Another shortcoming of most rate-responsive pacemakers is that theyentail complex set-up procedures. (See, for example, Application SerialNo. 150,038, filed on Jan. 29, 1988 in the name of Nappholz et al andentitled "Minute Volume Rate-Responsive Pacemaker".) There are otherdisadvantages with present-day approaches to rate-responsive pacing, andthey will become apparent when the advantages of the subject inventionare described below.

It is a general object of my invention to provide a rate-responsivepacemaker which overcomes the disadvantages of the general prior artapproach in which there is a one-to-one correspondence between pacingrate and RCP value. (The invention has more general applicability inthat it can be used in any automatic gain control system for relating acontrolled parameter to a controlling parameter.)

In accordance with the principles of my invention, the pacemaker is notprogrammed to pace at a particular rate for a particular value of theRCP. Instead, there is stored a function which represents the desiredrate distribution. Using discrete values, the function might call for arate of 70 beats per minute (bpm) to be the operative rate 40% of thetime, a rate of 80 bpm to be the operative rate for 10% of the time,etc. The pacemaker also generates a probability distribution function ofrecent RCP values. For example, it may be determined that over the lastmonth or so 25% of the time the RCP had a value of 5, 20% of the time ithad a value of 6, etc. From these two functions, two percentile rankingsare developed. Each percentile ranking is a cumulative distributionfunction. The function for the desired rate distribution might representthat the rate should be less than 60 bpm 10% of the time, less than 65bpm 30% of the time, less than 70 bpm 70% of the time, etc. Thepercentile ranking for the recent RCP values takes a similar form. Forexample, the RCP values over the last month may have been less than 315% of the time, less than 4 35% of the time, less than 5 60% of thetime, etc.

The two percentile rankings are then used to determine the pacing rateat any instant. The instantaneous RCP is measured and its percentileranking is determined from the percentile ranking table of recent RCPvalues. Using that percentile, the percentile ranking table for rates isconsulted. The rate corresponding to the previously determinedpercentile is the one used by the pacemaker.

The net result is that the rates at which the pacemaker paces have aprobability distribution which corresponds to the desired (programmed)rate distribution. The pacemaker is self-adapting, provided that thereis maintained a probability distribution function for recent RCP values.As the RCP values change with the administration of drugs and otherlong-term effects, the rate distribution is automatically mapped ontothe RCP value distribution.

Further objects, features and advantages of my invention will becomeapparent upon consideration of the following detailed description inconjunction with the drawing, in which:

FIG. 1 depicts the general prior art approach by which the rate of arate-responsive pacemaker is determined from the value of an RCP;

FIG. 2 represents two examples of a rate distribution which may bedesired by a physician;

FIG. 3 depicts a typical probability function for measured RCP values;

FIGS. 4A and 4B depict a typical RCP probability distribution function(histogram) and corresponding percentile ranking in the illustrativeembodiment of the invention;

FIGS. 5A and 5B depict a typical desired rate probability distributionfunction and percentile ranking in the illustrative embodiment of theinvention;

FIG. 6 depicts the percentile rankings of FIGS. 4B and 5B in table form,and further illustrates the steps involved in going from a measured RCPvalue to the setting of the pacing rate;

FIG. 7 is a block diagram of a pacemaker which implements the method ofmy invention;

FIG. 8 is a flow chart of the master processing loop of the pacemaker ofFIG. 7; and

FIG. 9 is a flow chart of the steps executed in the "Sample RCP andUpdate" block of FIG. 8.

The curve of FIG. 1 is the kind of function which characterizes atypical prior art rate-responsive pacemaker. For every value of therate-control parameter, there is a corresponding pacing rate. Thecharacteristic may be non-linear not only because the sensor itself maybe a non-linear device, but also because it may be desired that thepacing rate vary with RCP in a non-linear fashion. The basic problemwith providing a pacemaker with a built-in function of the typerepresented by FIG. 1 is that the function must change with time if thesame level of stress is always to result in the same pacing rate. Thisis because RCP values typically vary with the administration of drugs,changes in sensor sensitivity over time, etc.

The basis for the present invention is that there is a desiredprobability distribution of pacing rates, of the type depicted in FIG.2. A physician might desire that curve A characterize the paceroperation. What the curve represents is the probability of occurrence ofevery pacing rate. The probability function is comparable to theprobability distribution function of FIG. 5A, the latter representing adistribution in terms of discrete rates. Referring to FIG. 5A, it isassumed that the physician desires that there be only seven possiblerates, ranging from 60 to 90 bpm, in 5-bpm increments. The desired ratedistribution is such that a rate of 60 bpm will apply 10% of the time, arate of 65 bpm will apply 20% of the time, etc. Curves A and B of FIG. 2represent the same kind of thing, except they take into account allrates and the vertical axis represents probability (with the area undereach curve being equal to unity). In the illustrative embodiment of theinvention continuous curves of the type shown in FIG. 2 do not play apart. However, they are helpful in understanding the invention from aconceptual standpoint. The physician programs the pacemaker with valuessuch as those represented in FIG. 5A--a probability distributionfunction for seven specific rates. It should be noted that the sevenprobability values add up to 100% since it is assumed that only sevendiscrete values of rate are permitted. Two curves are shown in FIG. 2 inorder that it be appreciated that the physician may program differentprobability distribution functions into a pacemaker. Curve A might applyto an inactive patient, while curve B might apply to an active patient;in the latter case, the rate curve is skewed toward a higher range. Ingeneral, it is contemplated that there might be three probabilitydistribution functions which the physician might choose from inprogramming the pacemaker, for sedate, normal and active patients. Onesuch probability distribution function is shown in FIG. 5A. Whicheverfunction is programmed, that is the function which the physician desiresto apply to the patient for the life of the pacemaker, or at least untilit is re-programmed.

It should be noted that the probability distribution function in no waycorrelates rates and RCP values. All that is known from the probabilitydistribution function is that if a continuous record is kept of howoften the pacer operated at each of the possible rates, it will be foundthat each rate was in effect for a percentage of the total time whichcorresponds to that shown in FIG. 5A. How the desired distribution isachieved based upon measured values of RCP is what the invention is allabout.

Whereas FIG. 2 depicts the desired probability of rate, the curve ofFIG. 3 depicts the probability of RCP values as actually measured. Foreach value of RCP, there is a certain probability that it will bemeasured. The curve of FIG. 3 is not fixed as is the desired rateprobability occurring depends upon long-term changes, drug therapies,changes in sensor sensitivity, changes in patient lifestyle, etc. Inother words, a curve such as that shown in FIG. 3 represents the actualmeasurements of RCP, whereas curve A or curve B of FIG. 2 represents apermanent desired rate distribution.

FIG. 4A represents a probability distribution function of recent RCPvalues. In accordance with the principles of the invention, runningcounts are maintained of measured RCP values. Only discrete values ofRCP are considered; thus an RCP value such as 4.7 would be treated as avalue of 5. The probability distribution function values are shown inFIG. 4A in normalized fashion, that is, each value is a percentage withthe total adding up to 100%. What this means, for example, is that ofall possible RCP values over the last month or so, a value of 5 wasmeasured 25% of the time, a value of 6 was measured 20% of the time,etc. FIGS. 4A and 5A are comparable in that they are both normalized sothat the individual probabilities in each case add up to 100%.

While the probability distribution function of FIG. 4A was said to bebased on values measured over the last month, it is to be understoodthat the precise interval is not important. What is important is thatthe pacer have available some kind of record which shows how the RCPmeasurements are varying on a long-term basis. The question is how torelate a probability distribution function of RCP values to aprobability distribution function of desired rates, i.e., how to go froman instantaneously measured value of RCP to an instantaneous rate to beused based upon the two probability distribution functions.

The first step in relating RCP values to rate is to recast theprobability distribution functions of FIGS. 4A and 5A into percentilerankings of RCP values and rates, as shown in FIGS. 4B and 5B. Apercentile ranking is the same as a cumulative distribution function(CDF). Consider the RCP value of 5 in the probability distributionfunction of FIG. 4A. A value of 5 is measured 25% of the time, as shownin the figure. Similarly, a value of 3 is measured 15% of the time and avalue of 4 is measured 20% of the time. What this means is that valuesof 3, 4 or 5 are measured 60% of the time. That is the way in which apercentile ranking of 60 is developed for an RCP value of 5 in FIG. 4B.The percentile ranking of the highest possible RCP, a value of 8, isnecessarily 100 because all measured RCP values are less than or equalto 8. A percentile ranking is necessarily a monotonically increasingfunction. In a similar manner the percentile ranking of the desiredrates can be derived from the corresponding probability distributionfunction, although the percentile ranking can actually be programmed inthe pacer without having to go through the mathematical manipulationstarting with a probability distribution function of rates. Referring toFIGS. 5A and 5B, for example, rates of 75 bpm or less occur 80% of thetime (as shown in FIG. 5B), and this value is derived by adding togetherthe four individual probabilities (10%, 20%, 40% and 10%) for the fourrates which are equal to 75 bpm or less in FIG. 5A.

Given the percentile rankings of RCP and rate, one is mapped onto theother in accordance with the principles of my invention. Both aremonotonically increasing functions and it is relatively simple todetermine the rate which should apply for any measured value ofRCP--even though the overall range of RCP values, and the probabilitydistribution within that range, vary with time. The steps involved areshown in FIG. 6. Two tables are included in the drawing. The table onthe left is derived from the percentile ranking of FIG. 4B, and thetable on the right is derived from the percentile ranking of FIG. 5B.For example, referring to FIG. 4B it will be seen that RCP values equalto or less than 6 have been measured (over the last month or so) 80% ofthe time. That is why the percentage column in the RCP percentileranking table includes a value of 80 for an RCP value of 6 or less. Bothtables are simply another way of representing the percentile rankings ofFIGS. 4B and 5B.

The rate which is set in the pacemaker is derived in the following way,with reference to a particular example. In the following discussion, thesteps of the method are shown by the circled digits 1-5. The first stepinvolves measuring the instantaneous value of the RCP, something whichis done in every rate-responsive pacemaker. [As mentioned above, theparticular RCP is of no moment insofar as the subject invention isconcerned, although the assignee of this application markets arate-responsive pacemaker in which the RCP is respiratory minutevolume.]It is assumed in the example that the measured value of RCP is5. In the case illustrated, all RCP values are to the nearest integer,and all rate values are to the nearest multiple of 5 bpm. The RCPpercentile ranking table is consulted and in step 2 it is determinedthat the pacemaker has been operating over the last month or so suchthat values of RCP of 5 or less have been measured 60% of the time.

In the third step, this percentile value of 60 is used as an entry intothe rate percentile ranking table. There is no percentile value of 60since the values derived from FIG. 5B in the illustrated example have atable entry of 70 following a table entry of 30. The next highest tableentry is selected, 70 in this case. It is known from the rate percentileranking table that a rate equal to or less than 70 bpm is desired 70% ofthe time. In the last step, the pacer is set to operate at a rate of 70bpm.

It is in step 3 that the correspondence is established between the RCPpercentile ranking and the desired rate percentile ranking. It is notpossible to relate to selves. Referring to FIG. 4A, it will be seen thatRCP values of 4 and 6 both occur 20% of the time. Referring to FIG. 5A,a rate of 65 is desired by the physician to apply 20% of the time. Sinceany rate versus RCP curve such as that shown in FIG. 1 is generallymonotonically increasing as shown, or decreasing, it would not bepossible to relate only one of the RCP values of 4 and 6 to the 65-bpmrate. This is not to say that with monotonically increasing ordecreasing functions several values of RCP will not map onto the samerate. For example, referring to FIG. 6 it will be seen that RCP valuesof both 4 and 5 map to a rate of 70 bpm. (The percentile ranking for anRCP of 4 is 35%, this rate percentile ranking table of FIG. 6, andconsequently in the fourth step a rate of 70 bpm is once again selected) But this is simply a matter of quantization. Referring to

FIG. 5A it will be seen that rates of 65 and 70 bpm are desired a totalof 60% of the time. It is therefore to be expected that multiple valuesof RCP will map onto the 65-70 bpm region of the rate percentile table.If the probability distribution function of desired rate involvessmaller discrete steps, then there will be fewer "big jumps". It isapparent, for example, that because a rate of 70 bpm is to occur 40% ofthe time, there necessarily has to be a 40% jump in the rate percentileranking table as shown in FIG. 6. Obviously the jumps would be muchsmaller if rates of 67, 69, 71 and 73 were each to occur 10% of thetime.

One question is why in step 3 of FIG. 6 an entry is made to thepercentile ranking of 70 rather than a percentile ranking of 30. It isnot because the value of 60 taken from the RCP percentile ranking tableis closer to 70 than to 30. Were the RCP value measured equal to 4 and apercentile ranking of 35 derived from the left table of FIG. 6 in step2, the entry to the rate percentile ranking table would still be to the70 line rather than the 30 line, even though 35 is closer to 30 than to70. The reason has to do with the mapping rationale. In the exampleshown in FIG. 6, an RCP value exists such that this value and lesservalues have been obtained 60% of the time over some relatively longinterval. What is therefore desired is a rate such that that rate andslower rates similarly are desired 60% of the time. Were the ratepercentile ranking table of FIG. 6 entered at the 30 level, for which arate of 65 bpm would be set, what it would mean is that a rate wasselected such that that rate and lower values are desired 30% of thetime. That does not correspond to RCP values which have been measured60% of the time. But an entry to the rate percentile ranking table atthe 70 line means that the rate which will be set and all lower ratesare desired 70% of the time. This necessarily means that they aredesired at least 60% of the time, and this corresponds to the range ofRCP values (3-5) which have been measured 60% of the time.

Referring to FIG. 6, it will be seen that for a measured RCP value of 3,the percentile ranking is 15. In step 3 of the method of the invention,the rate percentile ranking table is entered between the 10 and 30percentiles, and this means that the 30 line is selected, i.e., thelowest rate which can be set is 65 bpm even though the physicianincluded a 60-bpm rate in his rate distribution. With finerquantization, however, it is likely that such a situation will notarise. Furthermore, it is certainly possible that in the future RCPvalues of 3 or less will be represented less than 15% of the time. Forexample, suppose that they exist only 8% of the time. In such a case,the first entry in the RCP percentile ranking table will be less than 8,and for any value of RCP of 3 or less a rate of 60 bpm will be selectedfrom the rate percentile ranking table.

The block diagram of FIG. 7 depicts the manner in which a pacemaker canbe constructed to implement the subject invention. A sensor input isapplied over line 8 to RCP sampler 10. The sensor input may be achemical measurement, an electrical parameter or even the signal pickedup by a pacemaker electrode (shown by the numeral 35). The RCP sampler10 simply delivers periodic samples of the RCP to microprocessor 15. Themicroprocessor cooperates with memory 20 to derive its operatinginstructions and for storing data. The microprocessor senses cardiacpotentials amplified by amplifier 30, and similarly causes pulsegenerator 25 to generate a pacing stimulus when it is needed.

The illustrative embodiment of the invention is a VVI pacer; it can bestandard in all respects except that its escape interval is adjusted inaccordance with the current value of the RCP. The master processing loopflow chart is shown in FIG. 8. At the top, a test for heartbeat sensingis shown. If a heartbeat is sensed, the pace timer is reset so that anew escape interval can be timed. However, if a heartbeat is not sensed,a check is made whether the time which has elapsed since the lastresetting of the timer is greater than the escape interval. If it is, itis an indication that a stimulus is required, and two steps now takeplace. First, the pace timer is reset so that another cycle of operationcan begin. Second, the patient is paced by causing pulse generator 25 onFIG. 7 to operate.

Next in the flow chart is a test whether the RCP sample timer exceedsfive seconds. In the illustrative embodiment of the invention, RCPsamples are taken every five seconds. As long as five seconds have notelapsed since the last sample was taken, the system simply moves on tothe sense step at the top of the flow chart. But if five seconds havegone by, the first thing that is done is to reset the RCP sample timerin preparation for another cycle. An RCP sample is then taken andvarious updating operations take place. The box labeled "sample RCP andupdate" on the flow chart of FIG. 8 is shown in detail in FIG. 9; it isin the flow chart of FIG. 9 that the various steps described withreference to FIGS. 4-6 are carried out.

After an RCP sample is taken, as shown at the top of FIG. 9, a check ismade whether the RCP percentile update timer exceeds 3.7 hours. In theillustrative embodiment of the invention, the percentile ranking of FIG.4B is updated approximately six times per day. The timer is not exactlyfour hours because that would mean that six samples would be taken everyday, at the same six times every day. By taking slightly more than sixsamples in every 24-hour period, the sample values which are stored aremore representative of all RCP values, with each time of day being givenequal importance.

If it is time for the percentile ranking of FIG. 4B to be updated, theRCP percentile update timer is reset so that another sample will be usedto update the ranking 3.7 hours from now. Then the oldest RCP sample ina 200-location memory is replaced with the current sample. The systemstores the most recent 200 samples. If approximately six samples aretaken each day, the samples stored represent the RCP values measuredduring the last month of pacer operation.

Once the current sample replaces the oldest sample in the 200-locationmemory, the new percentile ranking is computed. How this is done isdescribed in the next step on FIG. 9. Although the probabilitydistribution function of FIG. 4A need not actually be derived, andinstead the percentile ranking of FIG. 4B can be derived directly fromthe 200 stored samples, it is convenient to consider the processing intwo steps. First, as shown in FIG. 4A, a count is taken of the sampleswhich correspond to each discrete value of RCP for which a count ismaintained; the total is divided by two to derive the probabilitydistribution function value for that particular RCP. The percentileranking of FIG. 48 for each value of RCP is then computed simply byadding together the probability of that value of RCP and theprobabilities of all RCPs of lesser value. In effect, the percentileranking which is derived represents the history of RCP measurements overthe last month or so.

The next step in FIG. 9 describes what is shown in FIG. 6 of thedrawing. It should be appreciated that although RCP samples are used toupdate the percentile ranking only approximately once every four hours,an RCP sample is taken every five seconds, as shown in FIG. 8, and everyfive seconds the pacing rate is adjusted in accordance with the stepsshown in FIG. 6. The last step in the flow chart of FIG. 9 simplyentails setting the escape interval so that it is equal to thereciprocal of the new rate; as is known in the art, the escape intervalis simply the reciprocal of the rate.

With this description in mind, there are several advantages of theinvention which are noteworthy. The first relates to the concern whichhas existed since the early days of pacemakers about the generation ofpacing pulses at rates which are excessively high. While a pacemakerusually includes a rate limiting circuit so that a maximum rate cannotbe exceeded, that does not necessarily prevent sustained pacing at themaximum rate. In the invention, however, no matter how "wild" the RCPvalues become, high pacing rates cannot be sustained. In effect there isa form of negative feedback; the pacer self-adapts to the RCP valuedistribution, even if all of the values are unusually high.

Another advantage relates to the fact that if a typical prior artrate-responsive pacemaker is not set up properly for a particularpatient, the rate-responsive capability of the device will generally beuseless. In the invention, however, not only is improper set-up oflittle concern, but there may be no need for set-up at all. Whatever theRCP values happen to be, and even if they are way too high or way toolow because of improper set-up, the RCP percentile ranking isautomatically mapped onto the percentile ranking of the desired ratedistribution. The set-up procedure described in the above-identifiedNappholz et al application requires measurements to be taken of the RCPwhile the patient is at rest and, after an interval of about an hour,when he suddenly starts to exercise strenuously. The RCP values aretelemetered from the pacer and used by the programmer to program thepacer. In the invention there is no need for all of this, as the pacerself-adapts to long-term changes in RCP measurements which do not relateto instantaneous physiological needs.

Another advantage pertains to the fact that prior art rate-responsivepacemakers pace at the nominal rate most of the time, with the rategoing up when the patient is subject to stress or when he exercises. Butthere is often no difference in rate when the patient is sitting in achair and when he is sleeping. If the set-up procedure involves ameasurement of the RCP which corresponds to the minimum rate when thepatient is at rest, there is generally no way to decrease the rate whenthe patient is sleeping. To do that would require that the minimum ratebe set so that it corresponds to a still lower value of RCP which mightbe measured while the patient is asleep. (Throughout this discussion itis assumed that increasing values of RCP correspond to increasing rates;obviously, the same remarks still apply if in a particular case aninverse relationship exists.) It might be possible to extrapolate, thatis, to measure the RCP value of a patient at rest and to compute what itshould be when he is asleep so that the computed value could be set tocorrespond to the minimum rate. However, extrapolations of this type areusually not accurate. In the invention, on the other hand, the RCPvalues which are stored include those taken while the patient issleeping. Those values go to make up the percentile ranking just as dothe other values, and thus those values also map onto the desired ratedistribution. Consequently, it is possible to provide truerate-responsive pacing over the entire gamut of patient activity.

A most significant advantage of the invention is that it is applicableto any rate control parameter. There is no need for different kinds ofprocessing depending on the particular RCP which is used. The entireproduct line of a manufacturer may provide the same kind of operation,whether the individual pacemakers use rate control parameters involvingtemperature, minute volume, stroke volume, etc. The RCP values arecompletely arbitrary in the sense that there is no predesignedcorrespondence between them and the desired rates. The RCP can even be anon-linear parameter without affecting the self-adaptation (as long aschanges in the RCP are monotonic). Even if the sensor is not workingproperly, whatever values of RCP actually exist have their percentileranking automatically mapped onto the percentile ranking of the desiredrate distribution. This is a remarkable result. What it means is that ifa sensor or the circuitry for processing the measured RCP signalsuddenly changes characteristics, that does not mean that the pacemakerno longer functions properly. It may take a month or so for the newpercentile ranking of RCP values to be mapped onto the percentileranking of the desired rate distribution, but once that takes place thepacemaker will operate as it did before. One obvious advantage of thisis that the RCP processing may be made logarithmic so that thesensitivity at low values may be increased.

In essence, the rate-responsive pacemaker of the invention exhibits apredetermined rate distribution, regardless of the distribution of therate-control parameter values. This does not mean that the pacemakerignores the parameter. On the contrary, the invention is a method oftransforming an arbitrarily distributed parameter into a rate with apredetermined distribution. Two properties characterize the pacer of theinvention. First, it is guaranteed to exhibit a programmed ratedistribution. Second, the pacing rate is guaranteed to changemonotonically with the measured RCP value so that as the RCP changes inany given direction, the pacing rate always changes in a correspondingdirection.

Although the invention has been described with reference to a particularembodiment, it is to be understood that this embodiment is merelyillustrative of the application of the principles of the invention. Forexample, instead of the percentile ranking of RCP values being basedupon values measured over the course of a month, they could be takenover a week and perhaps over as short an interval as one day. Also, inthose cases where the RCP is not monotonic in nature, it can betransformed into a monotonic parameter suitable for use in theinvention. It is known, for example, that blood temperature dips at theonset of demand, but then increases. An RCP based on temperature mightbe transformed to a new parameter which increases in response to asudden dip and also increases in response to an increase in temperature.In such a case it would be the transformed parameter which would betreated as the RCP whose values are ranked. Thus it is to be understoodthat numerous modifications may be made in the illustrative embodimentof the invention and other arrangements may be devised without departingfrom the spirit and scope of the invention.

I claim:
 1. A control system comprising means for measuring a value of acontrolling parameter; means for adjusting a controlled parameter; andcontrol means for (a) calculating a total percentage of time, over aninterval which is much longer than the response time of the system, thatsaid controlling parameter is equal to or less than each of at leastseveral values, (b) for representing a desired controlled-parameterdistribution which for each of different percentages of time indicatessaid controlled parameter equal to or greater than that which isdesired, and (c) responsive to a measured value of said controllingparameter, for relating said calculated total percentage of time forthat controlling parameter to said desired controlled parameter for thatpercentage of time and for causing said adjusting means to adjust saidcontrolled parameter to equal said desired controlled parameter for thatpercentage of time.
 2. A method for operating a control systemcomprising the steps of measuring a value of a controlling parameter;calculating a total percentage of time, over an interval which is muchlonger than the response time of the system, that said controllingparameter is equal to or less than each of at least several values;representing a desired controlled-parameter distribution which for eachof different percentages of time indicates a controlled parameter equalto or greater than that which is desired; and relating said calculatedtotal percentage of time for a measured value of said controllingparameter to said desired controlled parameter for that percentage oftime and adjusting said controlled parameter to equal said desiredcontrolled parameter for that percentage of time.
 3. A method foroperating a control system comprising the steps of periodicallymeasuring a value of a controlling parameter, calculating a firstfunction which represents a distribution of differentcontrolling-parameter values over a time interval which is substantiallygreater than the response time of the controls system, storing a secondfunction which represents a desired distribution of a controlledparameter, and relating said first and second functions to determine theinstantaneous controlled parameter applicable to a measuredcontrolling-parameter value.
 4. A method in accordance with claim 3wherein said first function is a percentile ranking ofcontrolling-parameter values, and said second function is a percentileranking of desired controlled parameter values.
 5. A method inaccordance with claim 4 wherein said applicable controlled parameter isdetermined by ascertaining from said first function the percentile rankfor a measured controlling-parameter value, and using a correspondingpercentile rank to ascertain the controlled parameter from said secondfunction.
 6. A method in accordance with claim 5 wherein saidcorresponding percentile rank is the one ascertained from said firstfunction or, if not present in the percentile ranking of said secondfunction, then the next highest percentile rank in said second function.